The present invention relates to a discharge excitation gas laser device used in the industrial field.
In recent years attention is paid to excimer lasers as new laser light sources for industrial use. An excimer laser is one of ultraviolet lasers, by which oscillation lines can be obtained at wavelengths from 353 nm to 193 nm by combining a rare gas such as krypton, xenon, etc. with a halogen gas such as fluorine, chlorine, etc. for a laser medium gas. Since it is laser medium ga possible to obtain a high output for a short wavelength region by using an excimer laser with respect to conventional laser device, it is expected to be used as a new light source in various fields such as industry, medical service, etc. In particular, in steps of fabricating conductor devices, demand therefor is rising rapidly to serve as a light source for replacing mercury lamps in fabricating super LSIS. Excimer laser devices can be classified, depending on the method for exciting the laser medium gas, into discharge excitating type, electron beam excitation type, X-ray excitation type, microwave excitation type, etc. Among them, discharge excitation type excimer laser devices are used often in the industrial field due to the fact that the construction thereof is simple, that a high repetition rate is possible and that the size thereof can be easily reduced.
Hereinbelow a prior art discharge excitation gas laser device will be explained.
FIG. 15 is a diagram indicating schematically the construction of a prior art discharge excitation high repetition rate excimer laser device. In FIG. 15, reference numeral 1 is a gas-tight vessel; 2a and 2b are a pair of main electrodes; 3 is a peaking condenser; 4 is a preionization gas; 9 is a ventilation fan; and 10 is a heat exchanger, these constituting a laser oscillation tube. Laser medium gas 11 is enclosed in the gas-tight vessel 1. A secondary circuit composed of the peaking condenser 3, the preionization gap 4 and the main electrodes 2a, 2b is connected with a primary circuit composed of a switch 6 consisting of a thyratron, etc., a charging condenser 5, a charging inductor 8 and a DC high voltage power supply 7 at points A and A'. 12 is an arrow indicating transfer current; 13 is an arrow indicating the direction of rotation of the ventilation fan 9; 14 is an arrow indicating gas flow generated by the ventilation fan 9; and 15 shows a discharge region.
Now the operation of the discharge excitation high repetition rate excimer laser device constructed as described above will be explained. At first, electric charge is stored in the charging condenser 5 by the DC high voltage power supply 7. The switch 6 is closed at a point of time where a predetermined amount of electric charge is stored in the charging condenser 5 and the potential difference between the two terminals thereof arrives at a certain value. When the switch 6 is closed, the potential at the point A increases in the negative direction with respect to the potential at the point A'. As the potential at the point A increases in the negative direction, dielectric breakdown is produced at the preionization gas 4. In this way preionization is produced between one of the main electrodes 2a and the other main electrode 2b. At the same time the transfer current 12 flows in the direction indicated by the arrow from the charging condenser 5 and electric charge is transferred to the peaking condenser 3. As electric charge is transferred to the peaking condenser 3, the potential difference between the main electrodes 2a and 2b increases. When it arrives at the discharge starting voltage, a DC pulse discharge is generated between the one main electrode 2a and the other main electrode 2b. Thus electric energy transferred to the peaking condenser 3 is injected in the discharge region 15. In this way the laser medium gas 11 in the discharge region 15 is excited in a high energy state by this DC pulse discharge so that a so-called inverted distribution is established. Energy thus stored in the laser medium gas 11 is outputted in the form of a laser beam by an optical resonator (not shown in the figure). In order to have a sustained laser beam suitable for a purpose of working, etc., a series of the operations described above may be effected with a high repetition rate.
However, since the laser medium gas 11 in the discharge region 15 is deteriorated gradually by the laser exciting discharge and thus discharge characteristics are worsened, the succeeding DC pulse discharge is unstable and no proper laser output can not be obtained, until the laser medium gas 11 between the main electrodes 2a and 2b is replaced by fresh gas by diffusion.
Consequently the ventilation fan 9 and the heat exchanger 10 are disposed in the gas-tight vessel 1 in a prior art device so as to form the gas flow 14 in the gas-tight vessel 1 by rotating the ventilation fan 9 in the direction of rotation 13 indicated by the arrow. The laser oscillation is produced with a high repetition rate, in order to take out a sustained laser beam, by replacing the laser medium gas 11 between the one main electrode 2a and the other main electrode 2b by fresh gas and by cooling and regenerating it.
However, the prior art construction described above had a problem that boundary layers are formed in the neighborhood of the surfaces of the one main electrode 2a and the other main electrode 2b and thus no satisfactory flow speed can be secured, even if the flow speed of the laser medium gas 11 between the main electrodes 2a and 2b in FIG. 15 is increased.
FIG. 16 is a schematical diagram for explaining the aspect of the formation of the boundary layers. In FIG. 16, hatched regions represent the boundary layers 16. Further FIG. 17 is a graph indicating an example of the distribution of the flow speed on a transversal cross-section of FIG. 16 along a line indicated by B--B'. As indicated in FIG. 17, the flow speed is lower in the neighborhood of the surfaces of the one main electrode 2a and the other main electrode 2b than in the central portion. Because of this phenomenon, among positive and negative ions generated by the laser excitation discharge positive ions are attracted by the cathode, while negative ions are attracted by the anode. That is, since the boundary layers 16 are formed in the neighborhood of the surfaces of the one main electrode 2a and the other main electrode 2b and the flow speed is low there, when the repetition frequency is high, the succeeding laser excitation discharge is started, before the laser medium gas 11 in the neighborhood of the surfaces of the one main electrode 2a and the other main electrode 2b is replaced completely by fresh gas. Therefore atoms, which are apt to be positively ionized, are distributed with a relatively high concentration in the neighborhood of the cathode, while atoms, which are apt to be negatively ionized, are distributed similarly in the neighborhood of the anode. Now this phenomenon is considered, taking a kripton fluoride (KrF) excimer laser as an example. Rare gas such as kripton, helium, etc., which is apt to be ionized positively, is distributed so as to have a great concentration gradient in the neighborhood of the cathode, i.e. the main electrode 2b, while fluorine, which is apt to be ionized negatively, is distributed similarly in the neighborhood of the anode, i.e. the main electrode 2a. As a result, the laser medium gas 11 between the main electrodes 2a and 2b is no more uniform and the uniformity of the laser excitation discharge is impaired. Therefore the excitation efficiency is lowered and the pulse energy of the laser beam is decreased. This means that at a high repetition rate oscillation the energy injected in the discharge region 15 should be increased, if it is desired to have a same pulse energy. However, when the energy density injected in the discharge region 15 is increased, deterioration of the laser medium gas 11 is remarkably accelerated. Further, for a discharge excitation high repetition rate excimer laser, stable glow discharge by DC pulse discharge is inevitable and such a high energy discharge causes ion bombardment on the one main electrode 2a and the other main electrode 2b. As described above, in a KrF laser there exist at a relatively high concentration of fluorine in the neighborhood of the surface of the one main electrode 2a serving as the anode and rare gas such as kripton, etc. in the neighborhood of the other main electrode 2b serving as the cathode. Consequently, when laser excitation discharge is generated, a number of fluorine ions collide with the anode 2a. Since fluorine is extremely reactive, in a state where fluorine ions are implanted in the one main electrode 2a, they react with metal constituting the surface of the electrode, e.g. nickel, which produces metal fluoride such as nickel fluoride, etc. The surface of the one main electrode 2a is locally protruded convexly by this metal fluoride. On the other hand, although rare gas colliding with the other main electrode 2b is scarcely reactive, since it sputters the surface of the electrode even at a kinetic energy as low as the laser excitation discharge energy, the surface of the other main electrode becomes concave. If the surfaces of the one main electrode 2a and the other main electrode 2b are locally deformed, the uniformity of the electric field is disturbed, which causes local concentration of the discharge. For this reason, not only the spatial uniformity of the laser beam is worsened, but also the life of the main electrodes 2a and 2b is remarkably shortened.
Consequently, in conventional devices, in order to be able to produce a high repetition rate oscillation, a measure is taken, by which the laser medium gas 11 is circulated with an extremely high speed. However, since influences of the boundary layers increase more and more so that they cannot be neglected, as the flow speed of the laser medium gas 11 increases, the necessary flow speed increase exponentially with increasing repetition frequency. As the result, the size of the ventilation fan 9 increases so that it occupies the major part of the laser device both spatially and on the electric power consumption.